The present invention generally relates to the detection of ionizing radiation, and particularly to the detection of X-rays.
More specifically, the invention relates to a method for spectrally resolved detection of ionizing radiation, and to a detector for spectrally resolved detection of ionizing radiation.
Gaseous detectors, in general, are very attractive at photon energies lower than approximately 10 keV. The main advantages of gaseous detectors are that they are cheap to manufacture compared to solid state detectors, and that they can employ gas multiplication to strongly (on orders of magnitude) amplify the signal amplitudes. However, at energies exceeding 10 keV the gaseous detectors are less attractive as the stopping power of the gas decreases rapidly with increased photon energy. This results in a heavily deteriorated spatial resolution due to extended tracks of so-called long-range electrons, which are created as a result of the X-ray absorption.
An improved spatial resolution is achieved by a gaseous detector for use in planar beam radiography, in which electrons released by interactions between photons and gas atoms can be extracted in a direction essentially perpendicular to the incident radiation. A detector of such a kind is described in our copending international application No. PCT/SE98/01873 entitled A method and a device for planar beam radiography and a radiation detector and filed on Oct. 19, 1998.
It is of interest to the measure the energy of X-ray photons. However, what is normally measured with energy sensitive X-ray detectors is the energy released when the X-ray interacts with the detector material. In this interaction one or more primary electrons are emitted from the atom with which the X-ray interacts. Each of these electrons carries a kinetic energy, which energy can be transferred to the material through further interactions between the electron and the atoms of the material e.g. in the form of charges (electrons in an insulator, and electron-hole pairs in a semiconductor) or light in a scintillating material.
The released charge, or produced light, is detected with an instrument and the peak of the resulted signal, or the integrated signal, is a measure of the kinetic energy of the primary electron(s) emitted. This signal is also used as a measure of the energy of the incoming X-ray photon.
However, the kinetic energy of the primary electron(s) produced is not directly proportional to the X-ray energy as the X-ray can interact with the absorbing material in a number of different ways.
If the X-ray photon interacts with the material through photoelectric effect the kinetic energy of the released primary electron is the energy of the X-ray minus the binding energy of the electron, which in turn depends on which orbital the electron originated from (i.e. the K-, L-, M-shell etc.). Often, the converter material also consists of several different elements with different binding energies.
Furthermore, the photoelectron can or cannot be accompanied by one or more Auger electrons being emitted when the hole in the atom is filled by an electron from an outer orbit. This Auger electron can have a number of different energies depending on the primary electron that was emitted and on the electron that takes its place.
The X-ray photon can also interact with the absorbing material through Compton scattering where an electron is emitted together with a new X-ray of lower energy than the first. The new X-ray can be detected in the detector or it can escape, thus giving different deposited energies. The Compton electron emitted can have any energy between null and nearly the total X-ray energy.
There are several more, but less probable, ways the X-ray can interact with the absorbing material. These effects result in an ambiguity in the correlation between the amplitude of the signal and the energy of the incoming radiation, which reduce the energy resolution.
Accordingly, it is an object of the present invention to provide a method for spectrally resolved detection of ionizing radiation that overcome some of the problems described above, and which can operate in a broad energy range of incoming radiation and which provides for an improved energy resolution.
It is in this respect a particular object of the invention to provide such a method that also provides for a high spatial resolution.
A further object of the present invention is to provide a method for spectrally resolved detection of ionizing radiation, which is effective, fast, accurate, reliable, easy to perform, and which can be implemented in a simple and cost effective way.
Still a further object of the invention is to provide a method for spectrally resolved detection of ionizing radiation, which is sensitive and can thus operate at very low radiation fluxes.
Yet a further object of the invention is to provide a detector for spectrally resolved detection of ionizing radiation, which can be given a length, in the direction of the incoming radiation, for achieving a desired stopping power, which makes it possible to detect a major portion of the incoming radiation.
Still a further object of the invention is to provide a method for spectrally resolved detection of ionizing radiation, in which electrons released by interactions between photons and gas atoms can be extracted in a direction essentially perpendicular to the incident radiation. Hereby it is possible to obtain particularly high spectral and spatial resolutions.
Yet a further object of the invention is to provide a method for spectrally resolved detection of any kind of ionizing radiation, including electromagnetic radiation as well as incident particles, including elementary particles.
It is also an object of the present invention to provide a device for spectrally resolved detection of ionizing radiation, which can operate in a broad energy range of incoming radiation and which provides for an improved energy resolution.
It is in this respect a particular object of the invention to provide such a device that also provides for a high spatial resolution.
A further object of the present invention is to provide a device for spectrally resolved detection of ionizing radiation, which is effective, fast, accurate, reliable, easy to install and use, and of low cost.
Still a further object of the invention is to provide a device for detection of ionizing radiation, which is sensitive and can thus operate at very low radiation fluxes.
Yet a further object of the invention is to provide a device for spectrally resolved detection of ionizing radiation, in which electrons released by interactions between photons and gas atoms can be extracted in a direction essentially perpendicular to the incident radiation. Hereby it is possible to obtain particularly high spectral and spatial resolutions.
Still a further object of the invention is to provide a device for spectrally resolved detection of any kind of ionizing radiation, including electromagnetic radiation as well as incident particles, including elementary particles.
These and other objects are, according to the present invention, attained by methods and devices as claimed in the appended claims.
The invention described is a technique for detecting the energy of the X-ray with fewer uncertainties than those described above, by measuring the depth at which the X-ray interacted in the absorbing material. With this technique it is not possible to determine the energy of a single X-ray, but the energy spectrum of an incoming X-ray flux can be determined with good accuracy.
Further characteristics of the invention and advantages thereof will be evident from the following detailed description of preferred embodiments of the invention, which are shown in the accompanying drawings.